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So the natural system in botany proposes to arrange plants in such a way as to give an idea of the plan of creation. Plants as they occur in nature are viewed as individuals resembling or differing from each other. Some individuals are so decidedly alike that we at once give them the same names. Thus a field of wheat is composed of numerous similar individuals which can be separated from each other, but cannot be distinguished by any permanent or marked difference. Although there may be some difference as regards size and other minor points, still we at once say that they are all stalks of wheat. Every grain of wheat when sown, produces a stalk of wheat, and these stalks yield grains which produce individuals like their parents. The shoots or buds given off from the base of wheat by tillering, also produce stalks of wheat. From considerations such as these, we derive our idea of what is called a species, which may be defined as an assemblage of individuals presenting certain characters in common, and derived from one original stock.
There are no doubt differences in the individuals of a species, depending on soil and on different conditions of heat, light, and moisture. But these differences are not incompatible with the idea of a common origin ; and moreover we find that there is always a tendency to return to the original type. What is called a variety, is an individual of a species exhibiting variations which are not in general of a permanent character, and which cannot be kept up in the natural state, or, in ordinary circumstances, by seed. By means of buds or slips such varieties may be continued, but if their seeds are sown in ordinary soil and left to grow wild, the plants tend to return to the specific type. In certain plants, such as cereal crops and culinary vegetables, varieties have been perpetuated from seed by the art of cultivation, and thus races are kept up by artificial means. But when the seeds of such plants are sown in ordinary circumstances, and the plants are allowed to grow wild, we then see that there is a return to the parent type. In illustration of this statement we may refer to ordinary vegetables, such as cabbage, cauliflower, brocoli, savoys, and curled greens, which are all derived from one stock or type (Brassica oleracea). This plant grows wild on our sea-shores in certain places, and when cultivated it assumes peculiar forms. Thus it forms a heart, as in ordinary cabbage ; its flower-stalks become thickened and shortened, as in cauliflower and brocoli ; or its cellular tissue is largely developed, so as to give rise to the curled appearance of greens. These varieties are continued by cultivation, and after a series of generations, the seeds of the varieties propagate, more or less completely, plants of a similar nature. But if they are allowed to grow wild, then in the progress of time the variations disappear, and the original type of the species is reverted to. The varieties of apples and pears are continued by the art of horticulture and the process of grafting, but the seeds of these plants, when allowed to grow wild, produce the original stock, viz., the crab-apple or crab-pear, whence all the varieties have been produced. All these facts show the permanence of species in nature, and are not in accordance with the ideas of those naturalists, who state that one species can be transmuted into another in the course of generations.
Having thus determined what species are, we proceed to group them together so as to form a genus. This is an assemblage of nearly related species, agreeing with one another in general structure and appearance more closely than they accord with other species. Thus the genus Rosa or rose includes plants which are obviously allied as regards their general character, their flower, and their fruit. All of them have a well-marked kind of fruit, as seen in the common hep. But under this genus there are numerous species, such as the Scotch Rose, the Dog Rose, the Sweet-briar, and the China Rose, which differ in minor characters, derived from their leaves and other organs. The genus Quercus or oak includes many species which differ in the form of their leaves and in other respects, but all agree in their catkins, and in the character of their fruit, called an acorn.
Again, in examining genera, it will be seen that some of them, such as oaks, hazels, beeches, and chestnuts, have a strong resemblance or family-likeness, more especially as bearing catkins, or being what is called amentiferous, and that they differ remarkably from such genera as firs and pines, which bear cones, and from maples, which bear a winged fruit called a samara. In this way we group genera so as to form what are called orders or families. While a genus then is a group of allied species, an order is a group of allied genera. Thus firs, spruces, pines, larches, cedars, are united together in an order denominated Coniferæ or cone--bearers; while genera having a fruit called a legume are associated under the order Leguminosä. Sub-orders, or subdivisions of an order, may be formed also so as to group genera belonging evidently to the same order, but having special points of alliance, and thus facilities may be afforded to the student of botany in his determination of species. Certain orders again are more nearly allied than others, and thus we can group them into still larger assemblages called classes. Subdivisions, of classes, called sub-classes, are also formed to facilitate reference. The great object is to bring together those plants which are allied to each other in the structure of all their important organs.
We thus arrive at the following divisions :
II. Order or Family.
In giving the name of a plant we express the genus and the species. Thus Rosa spinosissima is the Scotch rose, where Rosa is the generic name and spinosissima the specific or trivial name ; so also Rosa canina, the dog rose ; Rosa rubiginosa, the sweetbriar. In giving the characters of classes, orders, genera, and species, we enumerate those marks which are necessary to distinguish them from others in the same category. These are called essential characters, and the shorter they can be made the better. Classes are distinguished by important general points of structure, such as the presence or absence of cotyledons, and their number. Orders are distinguished by characters taken from the general structure of the flowers and fruit. In the generic character we notice the modification of the ordinal characters in a given genus ; while in the specific characters are included less important modifications of form, whether in the stem, leaves, or flowers, which serve to distinguish allied species.
The object of this scientific classification is to bring together plants which are allied in all essential points of structure, and thus to follow what may be supposed to be the system of nature or the plan of the Great Creator. To accomplish this requires a full knowledge of the structure of plants all over the world ; and hence the systems adopted cannot, in the present state of our knowledge of the flora of the globe, be looked upon as by any means complete. Moreover we shall find that our divisions will prove, in many cases, more or less temporary or artificial, because in nature there are seldom marked and evident boundaries between groups. One class or order seems to pass into another by insensible gradations. There are no abrupt transitions to suit our precise modes of defining groups. Hence, while we can point out great centres of classification, as it were, round which we can arrange the plants of the globe, we are constrained to admit the incompleteness of all our definitions and lines of demarcation. We meet with aberrant forms which will not suit themselves to our definite groups, and which in fact possess, in a certain degree, characters common to several. Such forms, however, are useful landmarks, and they assist us in seeing the alliances subsisting between different orders. They point out in a very instructive manner the Master Builder's plan ; and although they may puzzle the student who adheres rigidly to definitions given in books, they give to the advanced student of nature enlarged and instructive views of affinities, and lead him to take a comprehensive survey of the plan of creation. On such ideas of affinity and relationship the natural system of botany has been founded by such men as Ray, Jussieu, De Candolle, Brown, Endlicher, Lindley, and Hooker.
Let us endeavour now to give a view of the natural system. In doing this we must take into account the structure and conformation of all parts of plants :
Beginning with the elementary tissues, it will be observed that some plants have cellular tissue (Fig. 10, p. 216) only in their composition, others have vessels of various kinds (Figs. 11 and 12). Thus we have two natural divisions of cellular and vascular plants. The latter may again be subdivided into those having true unrollable spiral vessels, and those having only modifications of such vessels in the form of scalariform vascular tissue. Cellular plants are represented by sea-weeds, mushrooms, and lichens ; vascular plants with scalariform vessels by ferns ; and vascular plants with spiral vessels by ordinary trees, shrubs, and herbs.
If we now take the nutritive organs, we shall arrive also at marked divisions. The embryo plant may be cellular without cotyledons, or it may have one or two cotyledons. By this means we get the great primary divisions of acotyledonous plants (Fig. 8, p. 213), such as sea-weeds, ferns, &c. ; monocotyledonous plants (Fig. 7), as palms, lilies, and grasses ; and dicotyledonous plants (Fig. 6), as peas, beans, and the trees and shrubs of Britain.
Next take stems, and we have plants having cellular or thallogenous stems, others acrogenous, a third set endogenous, and a fourth exogenous (Figs. 13, 14, 15, pp. 216, 217). Leaves furnish the division into plants which are veinless, those having forked veins of the leaf, those having parallel venation, and those having reticulated venation gs. 16 and 17).
In the next place, consider the organs of reproduction ; and here we meet with flowering plants having proper flowers, and flowerless plants having no such organs. In flowering plants, the parts of the flower are arranged in threes in one great division, and in two, four, and five in another (Fig. 22, p. 223). Stamens, and pistils with their seeds, occur in flowering plants ; cells of a peculiar nature supply the place of these evident organs in flowerless plants. On carefully examining these characters, we shall find that we can form the following divisions, and thus constitute the basis of a natural classification :-
Adopting these general principles of structure and arrangement, and, moreover, taking into account the relative position of different organs, their separation or adhesion, their completeness or incompleteness, we are enabled to construct a plan of classification which has been denominated a Natural System. The following are the outlines of this plan A. PHANEROGAMOUS OR FLOWERING PLANTS, WITH COTYLEDONS,
STAMENS AND PISTILS, AND SEEDS. CLASS I. DICOTYLEDONES or EXOGENÆ, in which spiral
vessels are present; the stem is exogenous ; venation of leaves reticulated ; flowers with binary or quinary symmetry ; embryo dicotyledonous.